Centrifugation of a biological or chemical sample in order to separate sample components requires high angular velocities. A drive system of a centrifuge may be required to spin a sample-containing rotor at 100,000 revolutions per minute. Air friction along the surface of the rotor is one factor that limits the maximum speed that can be achieved using a given drive motor. This friction is referred to as "windage." Windage not only acts as a limiting factor to the maximum angular velocity of a rotor, but also generates thermal energy that potentially increases the temperature of the sample to be centrifugally separated.
To counteract the effect of windage on the temperature of the sample, centrifuge instruments may include a refrigeration system. For example, refrigeration coils may be placed in surface contact with the exterior of a chamber of a centrifuge.
The effects of windage on rotor speed and temperature can be reduced by at least partially evacuating the centrifuge chamber. A vacuum pump may be connected to the chamber to achieve atmospheric conditions that reduce windage. However, the connection of the vacuum pump may significantly influence the cost of the centrifugal instrument. Typically, the chamber includes a removable cover. One option is to connect the vacuum pump to the cover. As compared to attachment to a fixed structure, attachment to the movable cover requires an increase in the length of interconnect hoses and requires the use of vacuum seals that are able to withstand the periodic stresses caused by pressure differential on the cover.
Another concern in the attachment of a vacuum pump to a centrifuge housing is minimizing the risk of foreign matter entering the vacuum pump. Even with careful handling, a sample may spill into the enclosed chamber defined by the centrifuge housing. If the material enters the vacuum pump, the efficiency of the pump may be adversely affected. Moreover, the useful life of the pump may be reduced. U.S. Pat. No. 4,857,811 to Barrett et al. describes the use of a vacuum pump having an inlet at a bottom surface of an enclosed chamber. While this position of the inlet is cost-efficient, the position is one in which gravitational forces will cause spillage and other foreign matter to enter into the pump. Alternatively, the inlet may be at sidewalls of the chamber. However, with the rotation of the rotor, a sample may be thrown directly from the rotor to the inlet, or may indirectly reach the inlet by first striking a sidewall and then being forced to the inlet by vacuum forces and the swirl created by the rotating rotor.
U.S. Pat. No. 5,084,133 to Guy et al. teaches a centrifuge of the type referred to as an evaporator-concentrator. A small quantity of gas is temporarily and periodically admitted into the vessel. The admitted gas is heated by a resistor for the purposes of heating the specimens to be concentrated and accelerating the rate of evaporation. A vacuum pump is connected to a chamber by a path through a rotating hub. Because the orifice to the path through the rotor is directed upwardly to the heating resistor, the structure is susceptible to the entrance of spilled sample or other foreign matter into the vacuum system.
An object of the present invention is to provide a device for centrifugally separating a sample, wherein a chamber may be evacuated in a manner that does not render a vacuum system susceptible to the entrance of foreign matter.